The inventive concepts described herein relate to semiconductor memory devices, and more particularly, to multi-level cell (MLC) memory devices and related methods of operation.
Memory devices are generally referred to as either volatile or non-volatile. A volatile memory device is characterized by the loss of stored data at a power-off state, whereas in contrast a non-volatile memory device is characterized by the retention of stored data at a power-off state. Examples of non-volatile memory include read only memory (ROM), electrically erasable programmable ROM (EEPROM), and the like.
Developed from EEPROM technology, a flash memory device is a type of non-volatile memory in which erase operations are carried out in units of memory blocks, and program operations are carried out in units of bits.
Flash memory is programmed by controlling a threshold voltage state of a memory cell. However, the threshold voltage of a memory cell can vary or drift from a programmed threshold voltage. This can result from a variety of different causes, such as floating gate coupling, charge loss over time, and so on. Any such variation in the programmed threshold voltage of a memory cell can adversely impact the reliability of a read operation. This is particularly problematic in the case of multi-level cell memory devices in which the margin between adjacent threshold voltage states is relatively small.